The invention relates to oscillators, including fixed oscillators and varactor tuned oscillators (VCOs), operation in the millimeter wave range including frequencies greater than 30 GHz.
The invention includes application not only in existing oscillators, but also in spatial power combiners. Spatial power combining is an emerging technology for current and future electronic warfare, radar, missile guidance and communication transceiver applications. A spatial combiner overcomes the intrinsic limitations on the power generating capability of millimeter wave solid state continuous wave active devices by combining the output of a multiplicity of active devices.
In preferred form, the invention provides a solid state millimeter wave oscillator in lumped element circuit form uniquely including an integral short wire antenna. The antenna serves the dual function of radiating the energy generated and also impedance matching the load to the oscillator. The antenna provides a simple means to transform the output impedance level of the oscillator (typically about 50 ohms) to the higher impedance level of free space or a waveguide (i.e. greater than or equal to about 377 ohms) providing a load. (The impedence of free space is 377 ohms. The impedence of a waveguide is greater than that of free space.) The invention eliminates the need for a separate antenna in a spatial power combining application.
The invention also provides significant improvements in miniaturization of a solid state millimeter wave oscillator. The oscillator is sufficiently small that it can be used as an active antenna element in a multi-dimensional array of such active antennas. A performance advantage results from such uniquely small size. In active antenna arrays, a close spacing of elements (typically half wavelength, i.e. 0.118 inch at 50 GHz) is necessary to prevent the generation of major secondary radiation lobes. The oscillator inherently satisfies the criteria for close spacing by virtue of its miniature circuit size (e.g. 0.089 inch by 0.089 inch at 50 GHz). The miniature size follows from the use of a lumped element circuit embodiment and its direct connection to an integral thin wire or ribbon antenna, typically a quarter wavelength long (e.g. 0.005 inch wide by 0.0005 inch thick by 0.06 inch long at 50 GHz). An intermediate impedance matching section is not required, as is conventionally used to couple an oscillator to a load. The required impedance matching between the oscillator and its load is accomplished by appropriately shaping the antenna and changing its length.
Also in accordance with the invention, an oscillator is provided with an integral antenna as the output coupling and matching element that is sufficiently short that it precludes the known "long line" effect. This effect adversely affects the frequency stability of an oscillator. The "long line" effect occurs when an oscillator is not terminated in a matched load and some load reactance is present, J. Altman, Microwave Circuits, Van Nostrand Company, Inc. Princeton, N.J. 1964, page 247, F. Terman, Radio Engineering, McGraw Hill Book Company, Inc., 1947, pages 464-465, J. Markus, Electronics Dictionary, McGraw Hill Book Company, N.Y. 1966, page 370. A long line length (greater than a quarter wavelength) between an oscillator and an unmatched load can result in frequency jumping or turn-on at an undesired frequency. The longer the line length, the larger are the number of undesired frequencies at which the fixed oscillator or VCO can operate. A simple means to eliminate the "long line" effect is to use an isolation element, e.g. an isolator or buffer amplifier, between the oscillator output and the load. The use of an isolation element adds RF (radio frequency) loss, size and cost to the oscillator. This is especially burdensome in active antenna array or phased array applications, for example, where a large number of oscillators are used.
In the present invention, the "long line" effect is precluded by the short length of the integral antenna (nominally a quarter wavelength long) and the zero connection length that follows from its direct connection to the oscillator. The invention eliminates the need for an isolation element and its associated penalties of RF power loss, size and cost. The size of an isolation element, as required with other oscillator circuits, precludes the realization of an active antenna array with close active element spacing noted above.
Also in accordance with the invention, a VCO is provided for use as the active antenna element in a spatial power combiner that uniquely provides for higher output power, higher efficiency and higher side lobe suppression than realizable with the use of a fixed oscillator as the active antenna element. A conventional spatial power combiner uses fixed oscillators as the active device. The multiplicity of fixed oscillators are made frequency coherent by self injection locking. When all the fixed oscillators are injection locked, they will not be phase coherent since all fixed oscillators were not at the same frequency prior to injection locking, due to slight constructional and component differences. After locking, all the fixed oscillators will be at the same frequency but each will operate at a phase angle determined by the difference between its free running frequency and the injection locked frequency. The phase difference between the fixed oscillators will result in the composite output power of the combiner being less than the sum of the output power of the individual oscillators. The composite output power would be equal to the sum of the individual locked oscillators if all oscillators had the same phase.
The use of a VCO, rather than a fixed oscillator, as the active element provides a means to control the phase of each locked VCO and thereby increase the output power and combining efficiency of the array. If a VCO is frequency locked and then its tuning voltage is changed, the frequency will remain constant but the phase of the VCO output will change. Hence, after an array of VCO active antenna elements are injection locked, a change of the varactor voltage of each individual active element will provide the means to establish a common phase for all VCOs and hence maximum composite power output.
The phase control capability provided by the use of a VCO, rather than a fixed oscillator, as an active antenna element in an antenna array also provides a means to electronically control the spatial distribution of the radiated composite output. In a broadside array, for example, the active elements are spaced one half wavelength apart and should all be at zero relative phase. If the criteria of zero phase is not met, the radiation pattern of the composite output will exhibit degraded directional properties. The electronic phase control provided by the use of a VCO permits the phase correction required after injection locking to obtain a nondegraded radiation pattern.
The use of a VCO, rather than a fixed oscillator, as the active antenna element also provides frequency agility to the combiner. To change frequency, the VCOs would be unlocked, tuned to the new desired frequency, injection locked at the new frequency and then phase adjusted, as described, to obtain maximum composite output from the combiner and optimum radiation characteristics. Active phase shifting is discussed in L. Cohen, "Active Phase Shifters For The Millimeter And Microwave Bands", IEEE MTT-S Digest, 1984, pages 397-399.